And yet all of you manage to get in the same team on the next round. If you won't play the first round at all banner balance would not apply to the guys that joined and you probably get split. But screens show you were all on the same team, so yeah, you must have played the first round for at least some seconds.
This is a wrong statement.
The balance works as following: After the first round, with low HP doors and no balance the defenders almost always lose, so Greys decide to stick in the spectator;
When it mentions one of the team wins or loses then they get into a team, they won't lose their multiplier since they didn't play, but they are in-game, so the second round they get together in a grey team, which is way better than losing your x5 on a random round. They also won't get any XP and Gold from watching so it's not profitable either.
They did not abuse the balance/team joining system because they didn't play the first round. Only if they join a team, go spectator and then join a team again to get in the proper teams then it's exploiting.